There is still considerable controversy in the field of B lymphocyte maturation and triggering, mainly due to the lack of direct approaches to study these problems. We have identified a surface component, Lyb3, which is expressed selectively on a mature subclass of B cells. This component is absent on B cells of CBA/N mice. We have further shown that Lyb3 is involved in antigen-dependent triggering. I will use a specific antiserum against this Lyb3 surface component to isolate mature and immature subsets of B-lymphocytes from CBA/H mice and to study their immunobiological properties, compared to B cells of immunodeficient CBA/N mice. I will test these B cell subsets with respect to quantitative and qualitative antibody production in a response to thymus-dependent and thymus-independent antigens and with respect to the ease of tolerance induction. Ontological studies of the appearance of Lyb3 ion B cells will help delineate B cell maturation and diversification. Further, I will analyze the mechanism of antigen dependent triggering by anti-Lyb3. I will test its triggering capacity in combination with different antigenic signals and by its ability to induce a high avidity memory response in the absence of T cells. I will also characterize the metabolic events during this triggering reaction. The quantitative expression of Lyb3 on B cells at different stages of antigen-induced activation will provide insight in the possible role that Lyb3 plays in the immune function of the cell. Biochemical characterization of the Lyb3 molecule will be done with the goal of purifying enough Lyb3 for direct functional studies.